Objective:: The purpose of this study was to investigate the morphology and number of roots of Korean mandibular third molars, and to evaluate the prevalence of C-shaped, two-rooted, and three-rooted mandibular third molars using cone-beam computed tomography (CBCT). Materials and Methods:: Serial axial CBCT images of the mandibles were gathered from 137 Korean patients. The total number of roots in the mandibular third molars of these patients was measured, and both the incidence and the correlations between left- and right-side occurrences, as well as between males and females, were analyzed. Results:: Most of the mandibular third molars either had two roots (56.5%) or one root (37.9%). There was no significant difference regarding the incidence of the different types of roots according to gender (female versus male) or topology (right versus left side). A higher percentage (80.5%) of the patients had similar root morphology on both sides. Conclusion:: The morphology and number of 214 mandibular third molars were examined using CBCT. There was a high prevalence of two-rooted mandibular and one-rooted mandibular third molars from this Korean population. Even though the anatomical variations in the mandibular third molars may not be high, these data regarding the occurrence and morphology of the roots will provide useful information to dentists performing these procedures.